Morphology-Driven Nonwettability of Nanostructured BN Surfaces

Article pubs.acs.org/Langmuir

Morphology-Driven Nonwettability of Nanostructured BN Surfaces Amir Pakdel,* Yoshio Bando, and Dmitri Golberg* World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan S Supporting Information *

ABSTRACT: Designing geometrical structures is an effective route to tailoring the wettability of a surface. BN-based hierarchical nano- and microstructures, in particular, vertically aligned and randomly distributed tubes and cones, were synthesized and employed as a platform for studying the influence of surface morphology on their static and dynamic interactions with water droplets. The variation of the contact angle in different hierarchical BN films is attributed to the combined effects of surface roughness and partial liquid−solid contact at the interface. Moreover, the impact response of water droplets impinging on BN arrays with different wetting properties is distinct. In the case of superhydrophobic films, the water droplet bounces off the surface several times whereas in less hydrophobic films it does not rebound and remains pinned to the surface. These results provide a facile route for the selective preparation of hierarchical BN nanostructure array films and a better understanding of their tunable water-repelling behavior, for which a number of promising applications in microelectronics and optics can be envisaged.

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substrate.17−20 The wetting properties of BN nanotube and nanosheet films have been investigated in refs 14−17. However, besides the aforementioned few works, there has been no comprehensive study on BN hierarchical structures to explore the geometrical effects of such engineered surfaces in detail. In the present work, we synthesize various morphologies of BN nano- and microstructure films via a CVD technique using a mixture of boron and metal oxides (FeO and MgO) under ammonia (NH3) gas flow and investigate the effect of surface morphology on their wetting behavior.

INTRODUCTION Recently, nanostructured materials have opened up a new era in the nonwetting properties of surfaces and have motivated great interest in their potential applications in self-cleaning surfaces, protective coatings, antifog glasses, and stain-resistant materials. Well-aligned or patterned assemblies of 1D and 2D nanostructures have been capable of building synergetic multifunctionalities into integral systems with enhanced collective responses. Inspired by various plant leaves and insect wings in nature, superhydrophilic surfaces with water contact angles (CAs) smaller than 5° and superhydrophobic surfaces with water CAs larger than 150° have been manufactured on the basis of inorganic hierarchical nanostructures.1,2 Among these nanomaterials is layered boron nitride (BN), a wideband-gap (∼6 eV) compound with excellent thermal conductivity and superb chemical and mechanical stability. These properties promote the application of hierarchical BN nanostructures in ultraviolet light-emitting diodes (UV LEDs), thermally conductive polymer matrix composites, and inert superhydrophobic coatings and films.3 A variety of methods, such as arc discharge, laser ablation, chemical vapor deposition (CVD), self-assembly on singlecrystalline surfaces, chemical exfoliation, ball milling, and chemical blowing have been used to prepare various morphologies of BN crystals, including fullerenes, tubes, helixes, meshes, sheets, and disks.3−13 However, the controlled growth of these BN nanostructures on a surface to produce uniform films and coatings via a simple and effective synthesis process remained a challenge until recently.14−16 In this regard, the present authors utilized a flexible CVD method in which pure or hybridized BN nanosheets and nanotubes could be selectively synthesized as a homogeneous coating/film on a © XXXX American Chemical Society

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RESULTS AND DISCUSSION Different sets of tubular and conical BN nano/microstructure arrays are shown in Figures 1 and 2. The morphology of the asgrown films is greatly influenced by process variables (i.e., the synthesis temperature and catalyst content). Films of thin BN nanotubes were produced at 1200 and 1400 °C in randomly oriented (Figure 1a) and bushlike (Figure 1b) morphologies, respectively. The molar ratio of the precursor materials was 2:1:1 B/FeO/MgO.19 Thick tubular BN structures with submicrometer diameters (Figure 1c,d) were generated when the catalyst content in the precursor materials was doubled (i.e., 1:1:1 B/FeO/MgO). Moreover, conical BN structures with outer diameters ranging from tens of nanometers to ∼1 μm were grown via the same setup, but in this case, NH3 gas was introduced from Special Issue: Interfacial Nanoarchitectonics Received: February 1, 2013 Revised: April 5, 2013

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Figure 1. SEM images of BN films consisting of tubular nanostructures: (a, b) thin tubes in randomly distributed and bushlike morphologies and (c, d) thick tubes in randomly distributed and bushlike morphologies. Photographs of water droplets are shown in the bottom right corners.

Figure 2. SEM images of BN films consisting of conical nano- and microstructures: (a, b) mikes in randomly distributed and vertically standing morphologies and (c, d) funnels in bushlike and randomly distributed morphology. Photographs of water droplets are shown on the bottom right corners.

furnace temperature was augmented to 1200 and 1300 °C, respectively. Nano- and microfunnels (Figure 2c,d) were grown as the temperature was raised to 1400 °C. The observed

the beginning of the experiment at room temperature. Nanoand micromikes with random (Figure 2a) and vertical-tosubstrate (Figure 2b) orientations were synthesized when the B

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morphologies of the samples prepared at 1200 and 1300 °C were almost uniform throughout the entire film. However, the BN funnels prepared at 1400 °C had a bushlike morphology near the edges of the substrate (Figure 2c) and a randomly oriented morphology in the middle of the substrate (Figure 2d) possibly as a result of some subtle environmental changes at the former locations on the growth substrate. The molar ratio of the precursor materials for the growth of all conical structures was 2:1:1 B/FeO/MgO. The chemical composition of the BN products was verified by electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS) (Supporting Information). To understand the detailed experimental conditions and mechanisms responsible for the formation of these complex structures, further investigation is required. The introduction of different hierarchical structures and the alteration of the film morphology directly influence the surface wettability. Two theoretical models (Wenzel21 and Cassie− Baxter22) contain the basic guidelines for the study of nonideal flat surfaces. The Wenzel model assumes that water droplets penetrate surface asperities, and the Cassie−Baxter model predicts the suspension of droplets on top of asperities. The wetting behavior in the Cassie−Baxter model is described by eq 1 cos θapp = fls cos θs + flv cos θv

modulated from a random distribution to a bushlike arrangement of tubes, the three-dimensionality of the interface between water and BN becomes more prominent. Therefore, the enhancement of surface roughness added to the superhydrophobicity of the BN film. Comparatively, the static water CA of conical BN structures was very sensitive to the surface morphology. Nano- and micromikes with random orientations in Figure 2a displayed superhydrophobicity with a CA of ∼164°, whereas the vertically aligned mikes in Figure 2b were considerably less hydrophobic with a CA of ∼105°. In the latter case, the surface morphology is inadequate for superhydrophobicity purposes, presumably because of the densely packed morphology of the uprightstanding mikes. This could increase the portion of the surface in contact with water droplets compared to the case of thinly distributed mikes, which led to the CA decrease of the BN film. Nano- and microfunnels with bushlike and random orientations in Figure 2c,d displayed hydrophobicity with CAs of ∼109 and ∼137°, respectively. The transition from random orientation to bushlike morphology is accompanied by a notable decrease in the CA. That is, water could enter the peculiar shape of the funnels with open tips whereas it was repelled by external funnel surfaces. This could be due to the influence of the sign of the curvature on the wettability of the funnels. In fact, thermodynamic calculations show that when the radius of the substrate curvature becomes comparable to the range of action of the surface forces field an increase in the curvature for a convex surface results in an increase in the CA, whereas for a concave surface the wetting improves. Such behavior can lead to the wettability transition from the hydrophilic state of flat surfaces to the hydrophobic state of convex surfaces.23 However, considering the relatively hydrophilic nature of BN surfaces, it is presumable that capillary imbibitions of funnels took place because of intermolecular attractive forces between water and BN surfaces, in which case the observed CAs should correspond to the metastable wetting state. To further clarify the effect of morphology on the wetting properties of the BN films, Figure 3 represents a schematic illustration of the interface between water and different vertically standing BN nano- and microstructures. Figure 3a

(1)

in which cos θapp is the apparent liquid CA on a rough surface, f ls and f lv are the liquid−solid and liquid−vapor contact areas, respectively, divided by the apparent area covered by a liquid drop, and θs and θv are the corresponding CAs on smooth solid and vapor surfaces, respectively. The liquid−vapor contact angle θv is 180°, and f is the fraction of the projected area of the tops of the solids in contact with the liquid (with f lv = 1 − f ) and r is the roughness of the wetted solid area, which is defined as the ratio of the actual over the apparent surface area (r ≥ 1). Therefore, eq 1 can be rewritten as cos θapp = fls cos θs − flv = rf cos θs + f − 1 = f (r cos θs + 1) − 1

(2)

To investigate the wetting behavior of the present BN films, static water CA measurements were performed at room temperature. The shape of the water droplets on the films and the measured CAs are displayed in Figures 1 and 2. For each case, five images were recorded and analyzed. The typical error in CA measurements was ±3°. The measured CAs for tubular nano- and submicrostructures of BN were larger than 150°, an indication of their superhydrophobic behavior, in contrast to the 51° water CA on a smooth BN surface (reported by the present authors in ref 17.). Such an increase in CA on these nonwetted surfaces can be explained by Cassie− Baxter’s model as a result of the small fraction of solid in contact with the liquid ( f ) and the large surface roughness (r). Randomly distributed nano- and submicrotubes in Figure 1a,c exhibit similar CAs of 154 and 152°, respectively. However, when the arrangement of the tubes changes to a bushlike hierarchical structure, the CA rises to 162 and 165° in nanoand submicrotube films, respectively. That is because the bushlike morphology represents a multilevel rough surface that could trap a large amount of air at the interface between the liquid and the surface. In fact, when the surface morphology is

Figure 3. Schematic illustration of the interface between a water droplet and vertically standing BN structures: (a) nanotubes, (b) micromikes, and (c) submicrofunnels. C

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Figure 4. Snapshots of water droplets impinging on BN films consisting of (a) vertically standing nanofunnels and (b) nanotubes arranged in a bushlike morphology.

films, in contrast to the oscillation of pinned droplets on the hydrophobic ones. These properties promote the application of hierarchical BN structures as inert water-repellent films with tunable degrees of hydrophobicity. Should the CVD production process be scaled up, the present films can be utilized in diverse applications such as self-cleaning surfaces, nonfogging displays, and protective coatings against acid rain corrosion.

indicates the limited contact between bushlike nanotubes, which results in a large CA and superhydrophobic behavior. However, micromikes and microfunnels in Figure 3b,c show a more extensive contact at the interface between the solid and the liquid and thus less hydrophobicity. Figure 3c also depicts the suggested penetration of water into the funnels, which results in a smaller apparent CA. To highlight the distinction between hydrophobic and superhydrophobic BN films, their dynamic water-repelling behavior was studied using a free-falling water droplet on their surfaces. Figure 4a displays a series of images taken when the water droplet hit the vertically standing BN nanofunnel arrays. Initially, the droplet deformed profoundly on the film surface and then retracted. However, the peculiar shape and relatively moderate hydrophobicity of the BN nanofunnels pinned the droplet at the interface and prevented its complete rebound from the surface. Hence, the droplet came to rest on the film shortly after impact. Figure 4b, however, shows the collision of the free-falling water droplet with the superhydrophobic bushlike BN nanotube film. In contrast to the previous case, after the flattening of the water droplet into a discoid shape, it retracted and completely bounced back with no observable liquid residue on the surface. The droplet remained completely intact during the collision and did not splash or fragment into smaller droplets. This confirms the excellent water repellency of the BN nanotube bushes. In fact, the large CA of the film allows the droplet to store its kinetic energy in surface deformation and to rebound fully. The droplet therefore behaves as a spring whose stiffness is the surface tension of the liquid.24

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METHODS

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ASSOCIATED CONTENT

The CVD growth of the crystalline BN nanostructures was performed in an electric tube furnace, as described elsewhere.19 In brief, the precursor powders (B/FeO/MgO) were mechanically mixed in different molar ratios (1:1:1 and 2:1:1) and were positioned in an alumina combustion boat covered with a Si/SiO2 wafer. The boat was then set into an alumina test tube inside a vacuum chamber. The chamber was evacuated to ∼1 Torr, and then NH3 gas was introduced at a rate of 0.4 mL/min. The precursors were heated to 1200, 1300, and 1400 °C, held for 30 min, and then cooled to room temperature. The morphology of the films was studied by a field-emission scanning electron microscope (FE-SEM; Hitachi SU8000, Japan). The CA measurements were carried out by a sessile drop method using a deionized water droplet of 10 μL volume deposited onto the films by a microsyringe. A high-resolution optical microscope (Keyence VH-5000, Japan) equipped with WinROOF V5.03 analysis software was used to measure the water CA of the films. A high-resolution high-speed camera (Phantom Miro eX2; Vision Research Inc., USA) with Pcc (Phantom Camera Control) V2.14.727.0 software was utilized to investigate the dynamic behavior of a free-falling water droplet on the film’s surface.

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CONCLUSIONS Hierarchical BN films with various morphologies were produced by a CVD process. The broad range of structures, in particular, nano- and microtubes, mikes, and funnels, displayed different wetting properties. Vertically standing tubes formed superhydrophobic films with the water CA reaching ∼165°, whereas vertically standing mikes and funnels were much less hydrophobic with CAs of ∼105 and ∼109°, respectively. However, randomly oriented mikes, tubes, and funnels showed more effective nonwettability with CAs of ∼164, ∼152, and ∼137°, respectively. Because no chemical modification or treatment was applied to the BN films in this study, the different water-repelling behaviors were attributed to surface geometrical factors and were explained by the Cassie− Baxter model, which describes metastable wetting conditions. Moreover, the investigation of the impact response of water droplets impinging on the BN surfaces demonstrated the bouncing back of free droplets from the superhydrophobic

* Supporting Information S

Chemical composition characterization by EELS and EDS. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], GOLBERG.Dmitri@ nims.go.jp. Notes

The authors declare no competing financial interest. D

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(19) Pakdel, A.; Zhi, C. Y.; Bando, Y.; Nakayama, T.; Golberg, D. A Comprehensive Analysis of the CVD Growth of Boron Nitride Nanotubes. Nanotechnology 2012, 23, 215601. (20) Pakdel, A.; Wang, X.; Bando, Y.; Golberg, D. Nonwetting “White Graphene” Films. Acta Mater. 2013, 61, 1266−1273. (21) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988−994. (22) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (23) Boinovich, L.; Emelyanenko, A. The Prediction of Wettability of Curved Surfaces on the Basis of the Isotherms of the Disjoining Pressure. Colloids Surf., A 2011, 383, 10−16. (24) Callies, M.; Quere, D. On Water Repellency. Soft Matter 2005, 1, 55−61.

ACKNOWLEDGMENTS This work was supported by the WPI Center for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS; Tsukuba, Japan). We thank Nobby Tech. Ltd. (Tokyo, Japan) for providing the high-speed cameras. A.P. is grateful to Mr. Hiroshi Sonobe from Nobby Tech. Ltd. (Tokyo, Japan) for high-speed camera imaging and Drs. Mohammad Khazaei and Rhiannon Creasey for fruitful discussions.

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